I just started Dungeon Crawler Carl. In it he fights a Spanish speaking boss. The Spanish is not translated (so far) and if you can read it, it really changes the mood and impact of the entire scene.
Are there any other books that hide significant plot points behind the use of foreign language?
In my head, that scene is much darker than almost anything that follows. There's a lot where if you take a step back and think about it, ooof. But that's scene is the most inescapable gut punch (if you know some Spanish.)
Great series though, mixes the light heartedness with an eventually pretty heavy undertone.